This invention relates to a method for calibrating transducer type strain gages and more particularly to a method for accurately predetermining the individual apparent strain curve characteristics of strain gages prior to actual use.
A strain gage is a device which uses the change of electrical resistance of a wire under strain to measure stress. Because resistance changes with both strain and temperature, however, the apparent strain due to temperature change must be compensated for if accurate data is to be collected.
Inherent differences exist in the apparent strain curves of all resistance type strain gages. Because the differences are magnified at cryogenic temperatures, a resultant apparent strain arises when four arbitarily chosen strain gages are wired in a four-active-arm Wheatstone bridge circuit. Previously, corrections for apparent strain were made using a temperature sensitive wire placed internally in the Wheatstone bridge circuit. The correction wire has inherent problems, however, in that long lengths are often required and physical space on most transducers is limited. In addition, long pieces of this apparent strain correction wire are difficult to place in intimate contact with the strain-gaged surface and can cause erroneous loop data.
The wide operating temperature range of the National Transonic Facility at Langley Research Center makes it necessary to employ a force balance which has a minimized temperature induced output over the entire temperature range and which is capable of providing aerodynamic load data under cryogenic temperature conditions. Previous methods of correcting for temperature in the cryogenic temperature range are inaccurate due to their typically nonlinear, temperature induced response. It is preferable, therefore, in developing transducers with four-arm strain-gage bridges, to use strain gages wherein the individual apparent strain curves are accurately predetermined and matched. By matching strain gages prior to installation on transducers, the inaccuracies accompanying unmatched strain gages are greatly reduced, and the need to compensate for temperature induced strain gage output is virtually eliminated.
Accordingly, it is an object of this invention to provide a novel method for accurately predetermining the individual apparent strain curve characteristics of strain gages prior to actual use.
Another object of this invention is to provide a method of matching strain gages through data comparison of apparent strain response.
Still another object of the invention is to provide a method of matching strain gages whereby the gages are matched at points throughout the temperature excursion, as well as at the end point, thereby resulting in a decrease in nonlinear response and apparent strain.
Yet another object of the invention is to provide a method of matching strain gages whereby the initial accuracy for cryogenic transducers is greatly improved and less apparent strain correction wire is required.